In an air-conditioning system, functions of a heat exchanger of outdoor unit and indoor unit in a heating mode and in a refrigerating mode are just opposite with each other. When the air-conditioning system is operating in the heating mode, a low-pressure side of the outdoor unit is used as an evaporator, and the high-pressure side of the indoor unit is used as a condenser. When the air-conditioning system is operating in the refrigerating mode, the high-pressure side of the outdoor unit is used as the condenser, and the low-pressure side of the indoor unit is used as the evaporator.
In the refrigerating mode, refrigerant is condensed in the outdoor condenser, while, in the heating mode, the refrigerant is condensed in the indoor condenser. A size of the condenser determines a capacity of liquid refrigerant that the system can carry. In the heating mode, refrigerant capacity required by the system is little, and in the refrigerating mode, the refrigerant capacity required by the system is large. In one system, only a fixed capacity of refrigerant can generally be filled, therefore, in the heating mode, refrigerant not required is stored by configuring a liquid storage tank. In addition, when the air-conditioning system is cooling off, the high pressure of the outdoor unit is high, and the pressure of the liquid storage tank is relatively low, thus refrigerant of the system may be automatically transferred from the outdoor condenser to the liquid storage tank. In addition, when the air-conditioning system is in a refrigerating and oil returning mode, a frequency of a compressor of the outdoor unit is high, and opening of the throttling element of the indoor unit is large, thus the refrigerant will carry oil back to the outdoor unit at a high speed, and a large amount of refrigerant will also return to the liquid storage tank.
Therefore, when the system is switched from the heating mode to the refrigerating mode, the system refrigerating mode is started, and the system is switched from the refrigerating and oil returning mode to the refrigerating mode, a large amount of refrigerant may exist in the liquid storage tank, which easily causes low pressure to be high and refrigerant capacity of indoor unit to be less, which further leads to poorer refrigerating capacity of indoor unit.